Thermostable and thermoactive enzymes have great utility in academic research and industrial applications. The high stability of enzymes from thermophilic organisms enables technologies in molecular biology and diagnostics (the Polymerase Chain Reaction, for example). However, equivalent enzymes from thermophilic organisms are not always available. In these cases, directed evolution or computational methods can serve as a powerful tool to identify variants of mesophilic enzymes that confer thermostability. For example, current in vitro transcription methods are limited to reaction temperatures below 45° C. The typical viral RNA polymerases that carry out these reactions are not active at elevated temperatures, and there is a need to identify thermoactive and stable variants in order to carry out in vitro transcription reactions at elevated temperatures.